Same noun used with all three articles conveys different meaning:
- He is at a school. (We don't know which school.)
- He is at the school. (We know which school.)
- He is at school. (He is a teacher or student and is teaching or learning.)
- I buy the paper every day. (Newspaper)
- The professor presented a paper on recent findings about cancer cells. (A piece of writing)
- The children are learning the art of folding paper into decorative objects. (Material)
It helps to observe the following:
(1) When we use two or more adjectives to describe the same person or thing, we use the article only before the first adjective.
- He talked to a tall and blonde woman.
(He talked to a [one] woman who was tall and blonde.)
(2) When we use two or more adjectives to refer to more than one person or thing, we use the article before each adjective.
- He talked to a tall and a blonde woman.
(He talked to two women, one of whom was tall and the other was a blonde.)